Volatiles Versus Vegetation

Plants act as both global sources and sinks of highly reactive volatile organic compounds (VOCs). Models typically treat the uptake and degradation of these compounds as if they are mostly unreactive, like other more commonly studied biogenic gases such as ozone. A study by Karl et al. (p. 816, published online 21 October) suggests that VOCs may be more reactive than expected. By monitoring six field sites representing a range of deciduous ecosystems, several oxidized VOCs were found to have high deposition fluxes. Fumigation experiments in the laboratory confirmed that leaves are capable of oxidizing these compounds, and do so through an enzymatic detoxification or stress-response mechanism. Budgets for VOC flux in the atmosphere suggests that, on a global scale, plants may take up significant levels of VOCs in polluted regions, especially in the tropics.

Abstract

The biosphere is the major source and sink of nonmethane volatile organic compounds (VOCs) in the atmosphere. Gas-phase chemical reactions initiate the removal of these compounds from the atmosphere, which ultimately proceeds via deposition at the surface or direct oxidation to carbon monoxide or carbon dioxide. We performed ecosystem-scale flux measurements that show that the removal of oxygenated VOC via dry deposition is substantially larger than is currently assumed for deciduous ecosystems. Laboratory experiments indicate efficient enzymatic conversion and potential up-regulation of various stress-related genes, leading to enhanced uptake rates as a response to ozone and methyl vinyl ketone exposure or mechanical wounding. A revised scheme for the uptake of oxygenated VOCs, incorporated into a global chemistry-transport model, predicts appreciable regional changes in annual dry deposition fluxes.